There are several shortcomings to cylindrical products in which the bore surfaces are protected against abrasion and/or corrosion by the traditional method of centrifugal casting. In the traditional method a backer cylinder or liner shell is machined with the bore diameter oversized by one-eighth (1/8) to one-fourth (1/4) inch, the surfacing alloy in powdered form and in sufficient quantity to produce the desired surface layer thickness is distributed along the bore length, the ends are capped, and then the assembly is slowly rotated while being heated to a temperature above the melting point of the surfacing alloy, at which time the assembly is rapidly rotated while being externally cooled. The backer cylinder or liner shell is most often steel and the surfacing alloys most often iron-nickel-boron-carbon for abrasion resistance, nickel-cobalt-boron-silicon for corrosion resistance, or nickel-boron-silicon-tungsten carbide for both abrasion and corrosion resistance. If multi-lobed cylindrical products are required with protected bore surfaces, they are traditionally made by first centrifugally casting the surface alloy into the required number of single cylinders, slicing these lengthwise to provide the individual lobe shapes, and fusion welding the individual cylinders to form the multi-lobed product.
These methods include:
U.S. Pat. No. 4,596,282 describes injection molding and extrusion as processes currently employed for forming articles from plastics, wherein a heated fused plastic is forced under pressure into a mold cavity to solidify in the shape and size of the cavity. The charge of plastic material to be injected into the mold cavity must be heated and pressurized prior to injection, and in one approach, the plastic starting material is fed into a hollow cylinder having a screw therethrough. As the screw turns, the plastics forced into a heated zone of the cylinder ahead of a check ring on the head of the screw, so that a predetermined volume of heated, pressurized plastic is prepared for subsequent injection into the mold cavity by a forward movement of the screw and check ring within the cylinder.
Because the economic of injection molding depends upon attainment of long operation lives for the machinery, it is important that the inner lining of the cylinder have a high resistance to wear and corrosion by the heated plastic material. Should the inside of the cylinder wear away so that the inner diameter of the cylinder is enlarged, a clearance develops between the check ring and screw, and the inner wall of the cylinder so that the plastic material leaks back from the pressurized zone, with the result that the necessary pressure for injection molding will not be developed. The cylinder must then be refurbished or replaced, or a larger diameter check ring must be utilized and, in any event, the economic production process is interrupted.
As set forth in U.S. Pat. No. 3,836,341, extruder barrels and injection molding machine cylinders have frequently been cast in recent years with high-ferrous content alloy linings. These prior ferrous alloy linings typically have ambient temperature hardness in the range of 58-64 Rockwell C in other centrifugally cast state. While such high ferrous content linings demonstrate excellent wear resistance to abrasive fillers present in plastic compositions during extrusion or molding operations, some such linings have relatively poor corrosion-resistant qualities, especially under conditions which cause the plastic material being processed to partially decompose.
Some of the disadvantages of these methods include:
(1) The surfacing alloy must have a melting temperature substantially below that of the backer shell. This severely limits the choice of surfacing alloys.
(2) The slow melting and long time for solidification of the surfacing alloy results in significant dissolution of the metal of the backer or shell and diffusion of this metal into the surface alloy. Since the shell is usually steel, this results in significant iron contamination of the surfacing alloy. In many cases the corrosion resistance of the surface is severely impaired and in some cases, such as in cylinders used for extrusion of fluoropolymers, unacceptable degradation of the polymer occurs.
(3) In the case of multi-lobed cylinders, the weld can only be made in the backer metal and can only approach but not get too close to the surfacing alloy in order to avoid cracking and spalling of the surfacing layer. This results in a gap between the two or more lobes which gap extends into the substrate metal. This allows the product being processed to penetrate the gap causing abrasion, corrosion, and product degradation.
Another special problem arises because the corrosion-resistant problem associated with high ferrous content alloy linings is particularly acute when the resin feed contains halogenated polymers or copolymers, such as fluorocarbon resins. As a result of feed degradation, which is difficult, if not impossible, to control fully the extrudate in a relatively short period of time become contaminated with minute metallic particles. These metallic particles come from the corrosive attack on the barrel lining alloy and become dislodged from the lining and migrate into the extrudate during processing. The corrosive agents are believed to be the halogens and possibly other gaseous substances formed during injection molding and extrusion.
Although nickel ingredients have been employed in varying amounts in liner alloys (see, for example, U.S. Pat. No., 3,836,341) the prior art disclosures utilized it in conjunction with substantial amounts of iron or special ingredients such as the tungsten carbide. In short, the problem is that high ferrous content liner alloys have been known to be undesirable in corrosive atmospheres, while the non-ferrous alloys proposed heretofore which exhibit satisfactory corrosion resistance, are not as wear and abrasive resistant as the high ferrous content alloys linings. Moreover, the use of special and additional ingredients such as tungsten carbide can be too costly for certain commercial applications. Nickel-based alloys often lack the requisite hardness.
One object of the present invention is to provide a method for attaining bore surface protection of cylindrical products while avoiding the difficulties or shortcomings of the previous methods.
A further object of the present invention is to provide an essentially non-ferrous alloy which is suitable as a liner for injection molding or extrusion cylinders and which is particularly adaptable for use in the surface protecting method of the inventor.
Another object is to provide a nickel-based lining alloy with improved hardness, wear resistant and corrosion resistant characteristics.
A still further object is to provide a method of lining the inner diameter of a steel housing with a nickel-based alloy, said lined steel housing being employed as a cylindrical barrel in extrusion and injection molding devices.
These and other objects of the present invention will become apparent from the ensuing description and embodiments.